Apparatus for transmissive exposure of copy pairs

ABSTRACT

The proposed apparatus for transmissive exposure of copy pairs has a pressure device 15, which presses the copy pairs being transmitted against the glass plate 11 and which comprises a plurality of pressure shoes 25 which are disposed side by side in the transverse direction, with a flat base surface 27 pressing the copy pairs against the glass plate, each pressure shoe being resiliently acted upon centrally from below and being not only height-adjustable but also tiltable about a longitudinal axis and a transverse axis.

The present invention relates to the transmissive exposure of copypairs, and specifically, in particular, systems in which light-sensitivematerials with copy originals laid on them are exposed during a throughmovement below or, in the case of upward exposure, above a light source.In the case of offset printing, the copy original is usually aphotographic film and the light-sensitive material is an aluminumprinting plate. In the case of the blueprint process, the copy originalis, for example, a transparent drawing and the light-sensitive materialis a blueprint paper or a blueprint foil. In the latter case, namely theuse of transparent or light-transmitting blueprint foils, a plurality ofthe same can also be laid one on top of the other and exposedsimultaneously. If, for example, three blueprint foils are to be exposedsimultaneously, then, including the copy original, four material layersmust be moved through the path of the light beam.

The invention proceeds from an apparatus for the transmissive exposureof copy pairs, having a light source directed onto a glass plate and apressure device disposed on the other side of the glass plate, as wellas a respective pair of transport rollers in front of and behind thetransmission gap formed between the glass plate and the pressure device.Such an apparatus is known from EP-PS 0,117,294 B1.

The pressure device acts on the copy pairs in the course of transmissionto the glass plate and is intended to cause these to be held in intimatecontact during the exposure. If spacings remain as a result of includedair pockets, then unsharpnesses designated as "hollow copies" arecreated.

In the known construction, the pressure device is a foam layer whichrests on a rigid plate. At its lower surface, the upper end of acompression spring is supported, the lower end of which compressionspring acts on a tie rod with settable force, which tie rod acts on thelight source housing with the glass plate, so that the latterexperiences, going beyond its weight, an additional surface pressure onthe foam layer. A complete elimination of hollow copies is not achievedwith this construction. This also applies to the further mentionedpossibilities of the pressing-on by means of a multiplicity of closelydisposed springs or of a cushion standing at excess pressure, with whichin each instance a full-area pressing-on is to be achieved.

Similar considerations are also applicable to a construction which isknown from EP-PS 0,110,940 B1 and in which the glass plate is pressed bymeans of a pressure-settable springing towards a bearing plate, overwhich the copy pairs are drawn in the course of the transmissiveexposure.

DE-OS 1,522,850 further discloses a construction with upward exposure,in which a flat plate is pressed by resilient spring force downwardsagainst the glass plate; in this case, the flat pressure plate carries afoam layer, which is provided with a brush covering, e.g. a velourlayer. This construction also does not reliably prevent the occurrenceof hollow copies.

The object of the present invention is the provision of an apparatus fortransmissive exposure, with which an improved quality of the copyingresult is achieved and the occurrence of hollow copies is reliablyprevented.

Proceeding from the initially considered construction, the achievementof the set object takes place, according to the invention, in that thepressure device comprises a plurality of pressure shoes which aredisposed side by side in the transverse direction and which are pressedin the height direction by resilient spring force towards the glassplate, with a flat base surface, which are tiltable about thelongitudinal axis and transverse axis.

As a result of the division, according to the invention, of the pressurezone into a multiplicity of pressure surfaces, which are displaceable inthe direction of pressing-on and thus exhibit two tilting degrees offreedom, i.e. virtually a cardan joint, which pressure surfaces are inthis case rigid in themselves, in spite of dispensing with a surfacepressing which is entirely continuous in the transverse direction, thereis successfully provided the assurance of an intimate contact of thecopy pairs during their transmission through the exposure region on thisentire surface.

Expedient further developments of the invention are indicated in thesubclaims.

In claim 2, it is indicated how, in a manner which is simple from thepoint of view of design, the pressure shoes can be designed, with theability to be simultaneously pressed down by resilient spring force, tobe mounted tiltably about the longitudinal axis and the transverse axis,i.e. virtually with a cardan joint.

A further refinement of the invention resides in an oblique setting ofthe pressure shoes according to claims 3 and 4, in such a manner thatthe unavoidable gaps remaining between these are at an angle to thelongitudinal direction or direction of transmission. With such a design,no position of the copy pairs remains without supporting contact withthe base surfaces of the pressure shoes for longer than just a shortperiod of time, namely during a crossing of a gap. This design is to bepreferred especially where the two copy pairs are foils or papers whichare thin and have very little resistance to bending.

Furthermore, the design of the base surfaces themselves is expedientlydependent upon the nature of the processed material. In any event,according to claim 7, the base surface should exhibit a slip coating,i.e. a coating with the lowest possible coefficient of friction incooperation with the lower surface of the light-sensitive material. Ifthe latter is an aluminum pressure plate, then the provision--which isproposed according to claim 8 and which is known per se--with a bristlesurface is expedient. Especially advantageous is a flocking with shortnylon bristles having a length of approximately 0.8 mm.

If, for example in the case of the blueprint process, thelight-sensitive material is a foil or the like, the enamel coatingproposed in claim 9 is expedient. In general, it may be stated that inthe selection of the surfaces sliding relative to one another in eachinstance a particularly smooth surface such as that of the aluminumplate is to cooperate with a matt or rougher, possibly brush-typesurface of the pressure shoes, and matt and thus somewhat rougherphotographic materials are expediently paired with smooth and hardsurfaces of the pressure shoes.

The proposed transmissive exposure system proves to be particularlyuseful in the production of large-format blueprints. In, for example,the motor vehicle industry, or town planning, blueprints having a lengthof 6 m or more are required. In this case, the proposed design achievesnot only a high copy quality with the avoidance of regions ofunsharpness and hollow copies, but also the greatest accuracy to sizeand freedom from distortion. This even applies in the production ofmultiple blueprints in one pass, for example in the case of the exposureof two or more blueprint foils laid one on top of the other.

The invention is further explained hereinbelow by the description ofillustrative embodiments with reference to the accompanying drawings. Inthe drawings:

FIG. 1 diagrammatically shows a transmissive exposure machine in a sideelevation, partially cutaway;

FIG. 2 shows the transmissive exposure unit in an enlargedrepresentation;

FIG. 3 shows the top plan view according to the section line III--III ofFIG. 2;

FIG. 4 shows the cross section according to line IV--IV from FIG. 2;

FIG. 5 shows an enlarged representation of the detail V--V from FIG. 4;

FIG. 6 shows a perspective view of a pressure shoe;

FIG. 7 shows a perspective view of the transmissive exposure unit in thecase of the removal of the illumination device and of one of the uppertransport rollers;

FIG. 8 shows the top plan view, similar to that in FIG. 3, onto a designwith pressure shoes extending at an angle to the direction oftransmission.

The transmissive exposure unit designated as a whole by 3 isincorporated in a support housing 1 with a cover 2 which can be raisedor swung up. Said unit includes a light source 5 which is incorporatedin the cover 2, with a glass plate 11 which represents the lowerboundary thereof. The light source itself comprises a radiator 7 with areflector 9.

At part of the transmissive exposure unit which is incorporated in thesupport housing 1 is essentially a pressure device 15, which forms,between it and the glass plate 11, the transmission gap for the copypairs. A pair of entrance-side transport rollers 17 is disposed in frontof the transmission gap, and a pair of exit-side transport rollers 19 isdisposed behind the transmission gap. The respectively upper roller ofthese pairs belongs to that part of the unit which is incorporated inthe cover.

Again, in front of and behind these there is disposed an entrance-sidestage surface 21 and an exit-side stage surface 23. If required, anantistatic brush strip is disposed behind the upper roller of theexit-side pair of transport rollers 19, said brush strip being intendedto wipe off any possible static charges.

The pressure device 15 comprises a number of pressure shoes 25 which aresituated side by side in the transverse direction. Each pressure shoehas a substantially U-shaped cross section with a flat base surface 27,from the lateral edges of which side cheeks 29 are bent away at rightangles. In these there are formed in each instance two elongate holes31, which extend in the height direction and through which guide rods 33engage. This guidance leaves to the pressure shoes the possibility ofdisplacement in the height direction as well as a possibility of tiltingabout the longitudinal axis L and, to a sufficient extent, about thetransverse axis T (transversal axis).

The front and the rear edge of the base surface 27 merge, rounded off,into a short bent-away portion, which forms a run-up ramp 35.

With each pressure shoe 25 there is associated a pressure spring 37, theupper end of which acts on the center of the lower surface of the basesurface 27. The spring sits between the guide rods 23 and its lower endis supported on a spring counterbearing 39, which is height-adjustablein a suitable manner, so that the spring force can be set.

In the described manner, the entire pressure zone is divided into aplurality of pressure surfaces situated side by side in the form of thebase surfaces of the pressure shoes. With only small spacings there lieside by side in the transverse direction flat partial surfaces, whichwith reliably equal force press the exposure material being transmittedtowards the glass plate; this results in a very high copy quality andreliably prevents hollow copies.

In the embodiment shown in FIGS. 1 to 7, the base surfaces 27 aresubstantially rectangular and the unavoidable gaps between them extendin the longitudinal direction L, i.e. in the direction of transmissionof the copy pairs. In the case of copy pairs with a certain rigidity,especially if the material to be exposed comprises printing platescomposed of thin aluminum sheets, this in no way impairs the high copyquality achieved.

However, in the case of copy pairs which are thin and thus have verylittle resistance to bending, there may be a certain striation in theregion of the gaps between adjacent pressure shoes. The cause of thismay be that the intensive contact of the copy pairs does not indeedremain fully maintained in the gap region. Another reason may reside inthe differing light reflection in the gap regions on the one hand and inthe region of the base surfaces on the other hand. This has an effectwhere the light-sensitive copy pair to be exposed is alsolight-transmitting, as is the case with light-sensitive foils. In thesecases, a part of the light flux passes through the two copy pairs and isreflected by the base surfaces of the pressure shoes. It is possible toreduce this effect by blackening the base surfaces, in order to create alight absorption which is as complete as possible. Nevertheless, aremaining part of reflective light may lead to rather different exposureconditions as compared with the gap regions. In the event of conformityof the gap direction or transmission direction of the copy pairs, thismay become evident in the form of a striation.

A complete suppression even of these effects is achieved with the designshown in FIG. 8, in which the pressure shoes 25' haveparallelogram-shaped base surfaces 27', in such a manner that theirreach E extends at an angle α of preferably approximately 20° to thelongitudinal direction L indicating the direction of transmission. Withsuch a division of the total pressure surface, there are no positions ofthe copy pairs which remain for a relatively great length of time in theregion of a gap between adjacent pressure shoes. The fraction of theexposure time during which this takes place for certain regions has noeffect.

If the geometry of the parallelogram is selected so that the shorterconnecting line between two diametrically opposite corners subdividesthis parallelogram into two equal right-angled triangles and extends inthe longitudinal direction L, and each surface point of the copy pairswill cross one gap once in the course of transmission. This createsequality, for each point, of the ratio between the times during which itis crossing a gap and during which it is sliding, with physical support,on a base surface. This gives quite uniform pressure and exposureconditions in the transverse direction over the width.

I claim:
 1. Apparatus for transmissive exposure of copy pairs, having alight source (5) directed onto a glass plate (11) and a pressure device(15), which is disposed on the other side of the glass plate (11) andwhich is immovable in the direction of transmission as well as arespective pair of transport rollers (17, 19) in front of and behind thetransmission gap formed between glass plate (11) and pressure device(15), to push the copy pairs through the transmission gap, characterizedin that the pressure device (15) comprises a plurality of pressure shoes(25) which are disposed side by side in the transverse direction andwhich are pressed in the height direction by resilient spring forcetowards the glass plate (11), with a flat base surface (27), which aretiltable about an axis extending in the longitudinal direction (L) andan axis extending in the transverse direction (T).
 2. Apparatus fortransmissive exposure according to claim 1, characterized in that eachpressure shoe (25) exhibits a substantially U-shaped cross section, thelimbs of which are side cheeks (29) angled off from the lateral edges ofthe base surface (27) and in which in each instance two elongate holes(31) extending in the height direction are formed for the passage ofguide rods (33), the center of the lower surface of the base surface(27) being acted upon by a pressure spring (37).
 3. Apparatus accordingto claim 1 characterized in that the direction (E) of the reach of thebase surfaces (27') includes an angle (α) with the direction oftransmission (L) of the copy pairs.
 4. Apparatus according to claim 3,characterized in that the base surface (27') has a parallelogram shape,the long sides extending in the direction of the reach (E) and the shortsides extending in the transverse direction (T).
 5. Apparatus accordingto claim 1, characterized in that at least the front edge of the basesurface (27) of each pressure shoe (25) is bent over kink-free to form arun-up ramp (35).
 6. Apparatus according to claim 1, characterized inthat the counterbearings (39) supporting the lower ends of the springs(37) are displaceable in the height direction to set the spring force.7. Apparatus according to claim 1, characterized by a slip coating ofthe base surfaces (27) of the pressure shoes (25).
 8. Apparatusaccording to claim 7, characterized by a flocking of the base surfaces(27) with bristles of natural or synthetic fibers.
 9. Apparatusaccording to claim 7, characterized by an enamel coating of the basesurfaces (27).
 10. Apparatus according to claim 1, characterized in thatthe base surface (27) of the pressure shoes (25) is blackened.